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28 CHEMICAL ENGINEERING
revolving accelerate water supplied axially. The velocity is converted into pressure by discharge vanes (turbine pump) or by a spiral casing (volute pump). A single inlet makes for simple construction, especially in multi-stage pumps: a double inlet eliminates end thrust. For high heads, impellers may be connected in series, giving the multi-stage pumps. When driven by a steam turbine, speeds must usually be higher than when electrically driven. Designs vary in excellence: an efficiency of 0.70 to 0.80 usually implies fairly high first cost.
Power to Pump Water.—If C = cubic feet per minute, p = pressure (pounds per square inch) including static head, e = pump efficiency; then horsepower = Cp -T- 230e. Taking boiler horsepower = 120C, pump horsepower -f- boiler horsepower = p -*- 27,600e.
Efficiencies.—The leakage loss in a plunger pump may be under 5 per cent when the pump is new, increasing to 30 per cent or more after long wear with gritty water. Wear has little effect on the discharge of a centrifugal pump. Power efficiencies of plunger pumps running at rated capacity vary usually from 0.50 to 0.75 (somewhat less if outside packed), being greater for long-stroke pumps. If the pump varies its discharge by varying its speed, the efficiency holds up well at light loads. If the speed is constant and a trip or unloading valve is used, the efficiency at low discharge rates is very low.1
Injectors.—These form inexpensive and useful emergency boiler feeders. They should be supplied with cold water. The efficiency is unimportant, since all heat not employed in pumping or lost in radiation goes back to the boiler. Exhaust steam may be used to operate an injector feeding against boiler pressure. The discharge temperature will usually be around 160°. The weight of steam discharged through the injector is about Fp -f- 70 pounds per second, where F = minimum nozzle area, square inches and p = steam supply pressure, pounds per square inch, absolute. Under usual conditions, the weight of water delivered is about 10 times the weight of steam.
Feed-water Heaters.—Types include the open heater (Cochrane, Webster) in which the exhaust steam and water mix, and the closed heater, in which the water circulates in tubes surrounded by exhaust steam. Open heaters give slightly higher feed temperatures at the same back pressure, or slightly less back pressure at the same feed temperature. Filters, separators, etc., are provided to remove oil from the exhaust. The open heater forms a convenient receptacle for various drips, for the automatic introduction of any cold water make-up supply and for certain forms of feed-water treatment and purification. It may be of the "thoroughfare" type in which all exhaust steam in the pipe passes through the heater, or of the "draw in" type in which a branch from the auxiliary exhaust leads to the heater as a dead end. Open heaters must be located on the suction side of the feed pump and above (preferably 3 ft. or more above) the level of the feed-pump suction valves.
Closed heaters include the coil type with single or multiple coils, the plain straight tube type, the corrugated straight tube and the double tube with water passing through the annular space. Plain tubes must be so arranged with the tube sheets as to permit of expansion.
Operation of Heater.—In a closed heater:
1 The subject of "Pumps" is covered much more exhaustively in Section III, "The Transportation of Liquids."